Rear Wheel Alignment (Shim Question):
Thread Starter
Instructor
Joined: Apr 2006
Posts: 108
Likes: 22
From: Duvall WA
Rear Wheel Alignment (Shim Question):
In the process of setting Rear wheel alignment.
Is there a general "rule-of-thumb" regarding the rear shims?
IE: If an 1/8" shim is moved from the inside-pack to the outside-pack-
Will this increase the Toe-In an 1/8" ?
Regarding the FRONT-End Shims,
I have always used the following rule-of-thumb- to rough-in a setting.
Camber 1/16" Shim = +/ - 1/3 of a Degree
Caster 1/16" Shim = +/ - 1/2 of a Degree
Any of You Guys that have done Rear Home-Alignments may have figured this out?
Thanks for Any Suggestions
Is there a general "rule-of-thumb" regarding the rear shims?
IE: If an 1/8" shim is moved from the inside-pack to the outside-pack-
Will this increase the Toe-In an 1/8" ?
Regarding the FRONT-End Shims,
I have always used the following rule-of-thumb- to rough-in a setting.
Camber 1/16" Shim = +/ - 1/3 of a Degree
Caster 1/16" Shim = +/ - 1/2 of a Degree
Any of You Guys that have done Rear Home-Alignments may have figured this out?
Thanks for Any Suggestions
Burning Brakes
Joined: Jun 2005
Posts: 1,130
Likes: 106
From: laingsburg MI
2020 C3 of the Year Finalist - Modified
I just did this last week. An 1/8” shin moved the toe in 0.156”, an additional 0.030” shim moved the tie in .037”. as read by the alignment shop.
2026 Loser of the Year
Joined: Sep 2013
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From: New Or-leens Loo-z-anna
Another Rough-in check/measurement: place a straightedge against the inside of the frame on the passenger's side and measure the distance between the straightedge and the inboard side of the trailing arm near the rear edge. Write down the measurement.
Then use the same technique on the driver's side. The two measurements should be reasonably similar
Then use the same technique on the driver's side. The two measurements should be reasonably similar
Melting Slicks
Joined: Jun 2011
Posts: 3,114
Likes: 598
Because of how shims are installed (assembly required) they should first set the thrust angle, then adjust the toe. Once the thrust angle is set you can adjust (shim) equal on both sides to maintain the thrust angle of the rear. This assumes the camber has been already been set.
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Brett Foote
Safety Car
Joined: May 2004
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From: los altos hills california
Because of how shims are installed (assembly required) they should first set the thrust angle, then adjust the toe. Once the thrust angle is set you can adjust (shim) equal on both sides to maintain the thrust angle of the rear. This assumes the camber has been already been set.
With regard to "thrust angle" I have always set my rear toe-in in relation to the front wheels, as measured from the tire, using the string method, However my fronts and backs are always the same size, so if that is different, your statement about thrust angle is more accurate.
Also, as I half way remember, isn't the rear track slightly wider than the front track?
Melting Slicks
Joined: Jun 2011
Posts: 3,114
Likes: 598
eg when I measure rear thrust from the picture. I have a laser mounted to the rear wheel. It might be toe out or toe in, I don't care. I make sure left and right is the same reading. Then I setup to get the correct toe by moving equal shims width on both sides.
So I don't know how you would use strings to measure thrust, other than the string is parallel to the rear wheel and measure the distance of the string from the front hub ? Its very difficult to setup a string parallel when you only measure the distance of the wheel width and then measure 98" away from that point. eg if the string parallel is out by 1/64" (15" wheel) over a distance of 98" (wheel base) = 5/64 variance. That is poor accuracy. You need a fixed mounting on the rear wheel that extends past the front hub.
Safety Car
Joined: May 2004
Posts: 4,352
Likes: 1,581
From: los altos hills california
Yes the rear track width is wider, but how you setup rear thrust doesn't matter as long as each side is the same.
eg when I measure rear thrust from the picture. I have a laser mounted to the rear wheel. It might be toe out or toe in, I don't care. I make sure left and right is the same reading. Then I setup to get the correct toe by moving equal shims width on both sides.
So I don't know how you would use strings to measure thrust, other than the string is parallel to the rear wheel and measure the distance of the string from the front hub ? Its very difficult to setup a string parallel when you only measure the distance of the wheel width and then measure 98" away from that point. eg if the string parallel is out by 1/64" (15" wheel) over a distance of 98" (wheel base) = 5/64 variance. That is poor accuracy. You need a fixed mounting on the rear wheel that extends past the front hub.
eg when I measure rear thrust from the picture. I have a laser mounted to the rear wheel. It might be toe out or toe in, I don't care. I make sure left and right is the same reading. Then I setup to get the correct toe by moving equal shims width on both sides.
So I don't know how you would use strings to measure thrust, other than the string is parallel to the rear wheel and measure the distance of the string from the front hub ? Its very difficult to setup a string parallel when you only measure the distance of the wheel width and then measure 98" away from that point. eg if the string parallel is out by 1/64" (15" wheel) over a distance of 98" (wheel base) = 5/64 variance. That is poor accuracy. You need a fixed mounting on the rear wheel that extends past the front hub.
Here's a link that suggests that that is all that's needed
http://lshapedgarage.blogspot.com/20...asurement.html
Is this wrong somehow?
Racer
Joined: Feb 2012
Posts: 421
Likes: 27
From: Harrisburg PA
Not to high jack the thread, but I am replacing my trailing arms as well... when I take it to the shop for the alignment, should I expect them to set a '77 to stock alignment, or should there be an 'adjustment" for the fact that I am running wider/taller tires (25560/15)?
Melting Slicks
Joined: Jun 2011
Posts: 3,114
Likes: 598
Thanks. Didn't mean to imply that I measured thrust using the strings and have to admit I've never tried to measure thrust angle. Maybe someday! The thrust angle, as I understand it, is a perpendicular to a line through both rear axle centers and should align with the centerline of the car. I work till I get equal toe on both sides of the car.
Here's a link that suggests that that is all that's needed
http://lshapedgarage.blogspot.com/20...asurement.html
Is this wrong somehow?
Here's a link that suggests that that is all that's needed
http://lshapedgarage.blogspot.com/20...asurement.html
Is this wrong somehow?